New report on the State of Overwintering Sites in California

A report released today by the Xerces Society shows a sharp two-decade decline in the number of monarchs which overwinter along the California coast and prioritizes the top 50 overwintering sites most in need of conservation and management attention.

State of the Monarch Butterfly Overwintering Sites in California provides an analysis of western monarch population trends. A comparison of the average number of butterflies overwintering in California during the periods 1997–2001 and 2010–2014 shows a decline of 74 percent, a figure that is comparable to declines documented at monarch overwintering sites in Mexico.

The report presents a list of the 50 California overwintering sites most in need of protection and management, based on the severity of butterfly declines at each site and their importance to the remaining population. Listed sites span the monarch’s overwintering range along the Pacific coast and the East Bay of San Francisco, including Pismo Beach and Morro Bay Golf Course in San Luis Obispo County, Ellwood Main in Santa Barbara County, Pacific Grove Sanctuary in Monterey County, Lighthouse Field State Beach and Moran Lake in Santa Cruz County, and San Leandro Golf Course in Alameda County. Most of the 50 priority sites are publicly owned, however a substantial number of the largest sites are in private ownership.

In addition to documenting decline and prioritizing overwintering sites, the report also identifies important knowledge gaps and conservation issues regarding monarchs in the West and provides recommendations for management of overwintering groves. Intended for use by land managers, policy makers, restoration practitioners and researchers, this report raises awareness about the importance of California overwintering sites in conserving monarch butterflies and their migratory phenomenon in the West.

Monarch butterflies are distributed throughout North America. While eastern monarchs migrate to central Mexico for the winter, western monarchs spend the winter on the California coast as well as in central Mexico. Although the number of monarchs overwintering in Mexico experienced an uptick last year, according to a recent study by the U.S. Geological Survey, monarchs overwintering in Mexico have undergone an 84 percent decline from their population maximum in 1996. This decline is comparable to the decline in California over the same time period.

The loss of breeding habitat, especially milkweed, due to the increased use of genetically modified herbicide-resistant crops, is an important factor influencing the decline of monarchs in the eastern United States. Evidence for breeding habitat loss is less clear in the West, and the continent-wide decline in monarch numbers may be compounded by other stressors such as disease, increased frequency of drought driven by climate change and the widespread use of neonicotinoid insecticides, as well as destruction of their overwintering habitat.

Monarchs and their overwintering groves fall under a patchwork of legal protection in California, but are not comprehensively protected. Management to maintain the microclimate at these groves is often lacking. However, the monarch has recently been added as a Species of Greatest Conservation Need by multiple western states, including California, which will expand the ability for state agencies to engage in monarch conservation. California recently passed AB 559 which authorizes the California Department of Fish and Wildlife to take actions to conserve monarch butterflies and their habitats. In addition, the monarch butterfly is under review by the U.S. Fish and Wildlife Service for protection under the federal Endangered Species Act, with a final listing ruling expected in June 2019.

The data for this report comes from over one hundred committed volunteers who participate in the Xerces Society’s annual Western Monarch Thanksgiving Count, by monitoring the butterflies’ population size and habitat condition each fall around the Thanksgiving holiday. This report was completed with funding from the US Fish and Wildlife Service and Xerces Society members.

Read more about Xerces’ Monarch Conservation Campaign, including efforts to conserve overwintering sites in California and restore breeding habitat in key regions of the United States at http://www.xerces.org/monarchs/

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Monarch overwintering site location data provided by this website is for basic navigational purposes only and is not intended to be relied upon in situations where precise location information is needed or where erroneous, inaccurate or incomplete location data may lead to personal injury, death, or property or environmental damage. Neither we, nor any of our content providers, guarantee the availability, accuracy, reliability, or timeliness of location data displayed by our website.

Participants of this citizen science project assume all responsibility for following the rules and regulations of the areas in which they are surveying, including all notices of private property and no trespassing warnings. The Xerces Society encourages all participants to follow basic common sense and safety precautions when accessing and monitoring overwintering sites. Participants are ultimately responsible for their own safety and well-being.